A network as simple as two computers linked with a crossover cable has several points at which the network could fail. Large networks can have many points or nodes at which a single failure could disable the network.
The general rule for networks is that they should have no single point of failure. The broad factors that can bring down networks are:
Attacks: these include software attacks by various miscreants (e.g., malicious hackers, computer criminals) as well as physical destruction of facilities.
Failures: these are in no way deliberate, but range from human error in entering commands, bugs in network element executable code, failures of electronic components, and other things that involve deliberate human action or system design.
Accidents: Ranging from spilling coffee into a network element to a natural disaster or war that destroys a data center, these are largely unpredictable events. Survivability from severe accidents will require physically diverse, redundant facilities. Among the extreme protections against both accidents and attacks are airborne command posts and communications relays, which either are continuously in the air, or take off on warning. In like manner, systems of communications satellites may have standby spares in space, which can be activated and brought into the constellation.
Computer networks, large data grids, and information systems are becoming increasingly common place in several application areas such as banking, insurance, supply-chains, travel, enterprise intranets, etc. As the size of the network grows, it typically becomes more vulnerable to newer kinds of security threats that were hitherto implausible. One such vulnerability is due to distributed attacks. Distributed security threats are receiving increasing research interest due to the ubiquity of this problem. Some kinds of distributed attacks such as DDoS (Distributed Denial of Service) have been studied. They pose a class of attacks where global anomaly detection is triggered by exchanging information between nodes that have detected some local anomaly.
There is, however, another class of distributed attacks that pose a global threat, that is, a threat affecting more than one node in the network. This second class of distributed attacks includes events that appear normal or innocuous when viewed locally at any node. Absent global information, such threats are difficult or impossible to detect. However, because the events appear innocuous at every local node, there is little reason for the individual nodes to suspect anything wrong and exchange information.